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Name: Rohit
Status: student
Grade: 9-12
Country: India
Date: winter 2013-14

When high velocity particles travel faster than the speed of light in a medium, they create a blue flash. This is called Cherenkov Radiation. Why is it blue?


In Cherenkov radiation, the spectrum given off is more than just blue light. There is a continuous color spectrum, except that the intensity of radiation is higher in the blue range of color. In fact, the radiation is more intense in the ultraviolet spectrum, except that we cannot see in this range. A similar effect can be seen when we heat up a material such as a metal. If we have an electric stove, for example, as we turn up the heat, the stove element starts to glow deep red. It is radiating more than red light, but most of the light is in the red part of the (visible) spectrum, and our eyes perceive it as one color. As you turn up the heat, the peak of the radiation starts moving to the center of the color range, and we perceive the color as white. If you could turn the heat up even further (without burning the stove element), you would start to perceive a bluish color to the radiation. The peak of the radiation might be largely in the blue range (or even in the ultraviolet), and your eyes would interpret its color as bluish. Cherenkov radiation is similar, except if you plot the intensity of radiation with frequency (or inversely with wavelength) , it follows roughly a straight line in the visible band. Thus, our eyes perceive mostly blue colors, but if you look at the pictures of the radiation, you will notice a somewhat whitish color. Other colors are contributing as well, though not at such high intensities.

Kyle J. Bunch, PhD, PE

According to good old Wikipedia, the radiation is continuous across the spectrum. The higher the energy the more radiation is created. Apparently, only those energies that are high enough to produce visible light are also those with high enough energies to produce blue light. Try Wikipedia for a complete explanation of this phenomenon.

Hope this helps. Bob Avakian

Hi Rohit,

The speed of light in a vacuum is a universal constant (c). The speed at which light in a material is far less than c. The group speed of light in water is only 0.75c.

Cherenkov radiation results when a charged particle, most commonly a charged electron, is accelerated to faster than c in that dielectric medium. The particle typically is out of phase. It is a subset of the group velocity.

Electrons in the atoms of the medium will be excited to a higher energy level, and the atoms become polarized by the passing EM field of a charged particle.

Photons are emitted as an excited electrons restore themselves to the ground state after the excitation, giving off a blue light. The blue light is the photonic energy required for the electron to get to ground state. Hence the Cherenkov radiation.

Hoping this helps! Peter E. Hughes, Ph.D. Milford, NH

Hi Rohit,

Thanks for the question. I would refer you to the following Wikipedia webpage ( http://en.wikipedia.org/wiki/Frank%E2%80%93Tamm_formula) which describes the Frank-Tamm formula for predicting the emission spectra of Cherenkov radiation. Without evaluating the integral (which is a good exercise for physics students in college), a skilled math/science student can see that the intensity of the emission is proportional to the frequency (the Greek letter omega). So, higher frequencies are more intense than lower frequencies. Blue light has a higher frequency than red light, so the Cherenkov radiation appears more blue than red.

I hope this helps. Thanks Jeff Grell

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